Here are 1d20 possibilities to use as-is or to inspire more original ideas:
- Your character is from an impoverished noble family, seeking to regain the family fortune and reestablish the family lands and legacy.
- A beloved person in your character's life died, and a necromancer has promised to bring him or her back from the afterlife – for a price.
- You are a devoted member of your religion; every coin you can donate to your church will promote the interest of your god(s).
- You seek revenge against a powerful lord or lady who did some great wrong to you; but you'll need to buy an army to make that happen.
- Monster-hunting is a family trade; slaying monsters and taking their treasure is how members of your family have always made their fortune.
- You're simply ambitious and you seek to become the most powerful / feared / respected warrior / magician / religious leader in the land.
- You come from a poverty-ravaged region and wish to bring wealth back home to reinvigorate your homeland's economy.
- You seek a king's ransom – perhaps literally – to secure the release of someone important to you who has been imprisoned or enslaved.
- You grew up in abject poverty. You watched your parents and grandparents slave away for a lifetime and die scratching a living out of a patch of mud. That life is not for you.
- A plague is ravaging your homeland; the sages might be able to find a cure if only they had sufficient money to complete their research.
- Your homeland is the constant victim of raids from surrounding territories. If you could bring back enough money, your people could build fortifications and hire troops to defend them.
- Your homeland is ruled by a petty tyrant. With enough money and power you could overthrow the despot.
- Enemy raiders come to your homeland once per year demanding a tribute in gold or slaves. With enough gold, you can keep your people safe by paying them off . . . or even drive the raiders away once and for all.
- Monsters killed someone dear to you. You won't rest until you've extracted weregeld (or exacted blood revenge) from every monster in the region.
- Your homelands were conquered and occupied by a neighboring enemy. You need gold to take those lands back.
- Having seen the injustice of the world, you want to create your own utopian country, and you'll need gold to do it.
- You are an agent of the king / queen with orders to undermine threats to the region; you are to self-finance with loot, and hand over the rest to fill the royal coffers.
- An oracle declared you would grow up to be a great and powerful hero / mage / champion of the faith. You are simply fulfilling that destiny.
- You are obsessed ancient artifacts, books, magic items. No risk is too great to secure these precious items.
- You fought in a war that recently ended. Apart from adventuring, there's just nothing else to do with your skill set. Getting rich is an added bonus.